On Mon, 03 May 2021 10:23:20 +0200 Markus Armbruster <arm...@redhat.com> wrote:
> David Hildenbrand <da...@redhat.com> writes: > > > On 03.05.21 07:17, Thomas Huth wrote: > >> On 03/05/2021 06.58, Markus Armbruster wrote: > >>> Thomas Huth <th...@redhat.com> writes: > >>> > >>>> Clang unfortunately does not support generating code for the z900 > >>>> architecture level and starts with the z10 instead. Thus to be able > >>>> to support compiling with Clang, we have to check for the supported > >>>> compiler flags. The disadvantage is of course that the bios image > >>>> will only run with z10 guest CPUs upwards (which is what most people > >>>> use anyway), so just in case let's also emit a warning in that case. > >>> > >>> What happens when you try to use this bios with an old CPU anyway? > >> > >> Interesting question. I was expecting the guest to crash since it would be > >> using a CPU instruction that is not supported on the old CPU model. But I > >> just gave it a try, and there was no crash. The guest booted just fine. > >> Either Clang only emits instructions that work with the old z900 anyway, or > >> our emulation in QEMU is imprecise and we allow newer instructions to be > >> executed on old models, too. > > > > Yes, that's currently still done. We once thought about disabling that > > (there was a patch from Richard), but decided against it because -- back > > then -- the default QEMU model was still very basic and would have > > essentially disabled all more recent instructions as default. > > > > We can most probably do that change soon as we have a "fairly new" > > default QEMU CPU model. I can glue it to my z14 change. > > In case this makes the BIOS crash with old CPUs: when a guest refuses to > start because the BIOS was compiled the wrong way for it, configure > having told you so back then is not a nice user experience. Can we do > better, with reasonable effort? I fear the experience will be as bad as for any guest that is using features from a newer cpu level (i.e. random crashes when the guest actually tries to use that newer instruction.) I see two options: - Just try to start and hope that it works. - Deprecate any cpu model older than z10. Anyone have a better idea? I don't particularly like any of the two.
